The present invention relates to sensors for detecting the presence and/or strength of a magnetic field and, in particular, to a microelectromechanical system (MEMS) device providing for such measurements.
A magnetic field can be detected by noting its influence on a magnetic material or a current carrying wire. The former technique is used in a compass; the latter technique describes the common D""Arsonval movement used in electrical meters.
A Hall effect sensor may be used in applications that require a more compact and rugged sensor. Hall effect sensors detect the drift of charge carriers in a semiconductor material in the presence of a magnetic field. This drift causes a transverse polarization of that semiconductor which can be detected as a voltage.
Hall effect sensors are currently used in a number of applications including, switches, proximity sensors and magnetometers.
The present invention provides an alternative to the Hall effect sensor that is both rugged and small and promises improved sensitivity over, and more flexible implementation than the Hall effect sensor. In this regard, the invention provides a sensor constructed using a microelectromechanical system (MEMS) device, which may be mass-produced using integrated circuit techniques.
In the invention, a microscopic conductor extends between two terminals on a substrate and conducts a proof current. Deflection of the current carrying conductor under the influence of a magnetic field, caused by the Lorentz force, is measured by a detector coupled to the conductor to produce an output dependent on that deflection. The small size and mass of the flexible conductor make kilohertz or higher response speeds possible.
Generally, the output signal may be analog or digital depending on the selection of the detector and its processing circuitry. Optionally, the invention may include circuit elements providing the proof current on-board or the proof current may be supplied externally using a conventional current source. The flexible conductor may be a straight conductive segment for simple fabrication.
In one embodiment, a beam connects the flexible conductor to the detector and the beam and the flexible conductor may include a metalization layer on an insulating or semiconducting material. The metalization layer may be interrupted on the beam to provide electrical isolation between the detector and flexible conductor.
The detector may include a bias means producing a force resisting the Lorentz force on the flexible conductor. The bias means may be a mechanical element such as a MEMS spring or may be an electrical element such as an electrostatic, piezoelectric or thermal motor. The bias means may be either passive or active. If an electrical element is used, the invention may include a feedback circuit communicating with the bias means and responding to the output signal to vary a bias force resisting the Lorentz force on the flexible conductor. In this way, the bias means may receive feedback to provide improved linearity in the detection of magnetic fields.
In an alternative embodiment, the invention may include a compensation coil. A feedback circuit responding to the output signal may energize the compensation coil to oppose the crossing magnetic field. This approach provides the benefits of feedback without the need for an electrically actuable bias means, but with the need for a coil structure.
The invention may include a second flexible conductor and detector also producing an output signal and a combiner circuit combining the output signal from the first and second flexible conductors to reject detector signals not related to the strength of the crossing magnetic field B. The flexible conductors may be arranged to have countervailing current flows or may be oriented (in connection to their beams) in opposite directions so that the effect of environmental noise such as mechanical shock or vibration may be distinguished from the Lorentz forces.
The invention may further include a measurement conductor conducting a current to be measured and positioned adjacent to the flexible conductor so that the crossing magnetic field B is a magnetic field produced by the current through the measurement conductor. In this way, the present invention may be used to measure currents. A magnetic core may be used to concentrate the flux from the measurement conductor on the flexible conductor.
Alternatively, the invention may include a magnet providing the crossing magnetic field B and, in this way, may be employed as a proximity detector detecting distortions of the magnetic field caused by nearby ferromagnetic materials. The invention finds potential application in all applications currently served by Hall effect devices.
The foregoing features may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.